hey! i have played Gaia Crusader here, and have done a review (in french) here
http://membres.lycos.fr/wahrun/articles/gaiacrusaders.htm
The game is a Sengoku 3 0.5, with 5 characters (+2 playable bosses after the 3rd stage: a blue demon and a shaolin monk), some characters can transform(Kohen and Koou), and the game have a combo system who like just as the one in sengoku 3 (punch, punch, dash+punch, punsh,...).
A is punch
B is kick
C is jump (with double jump possible)
D is special attack
A+C is guard
A+B+C is super attack
the game is based on 3 element (sand, water, and fire), every character is specialised in 2. the special attack need an orb of element to be done. if the caracter is specialised in this element he do a giga attack (like the down, up, A+B in sengoku 3), if no he do a special attack (like the down, down+A/B in sengoku 3).
The game is very hard to beat, and there is a plenty of monsters (somes are redone in sengoku 3 like that bird-guy who is in gaia cusader a skeleton-bird-guy).
the story is the same as in sengoku 3.
this game isn't bad, but it's seems to old (in graphics and gameplay) for a 99 game.